Neonatal rat ventricular cardiomyocytes were used to investigate mechanisms underlying transient changes in intracellular free Ca 2+ concentration ([Ca 2+] i) evoked by pulsed infrared radiation (IR, 1862 nm). Fluorescence confocal microscopy revealed IR-evoked [Ca 2+] i events with each IR pulse (3-4 ms pulse -1, 9.1-11.6 J cm -2 pulse -1). IR-evoked [Ca 2+] i events were distinct from the relatively large spontaneous [Ca 2+] i transients, with IR-evoked events exhibiting smaller amplitudes (0.88 ΔF/F 0vs. 1.99 ΔF/F 0) and shorter time constants (τ= 0.64 s vs. 1.19 s, respectively). Both IR-evoked [Ca 2+] i events and spontaneous [Ca 2+] i transients could be entrained by the IR pulse (0.2-1 pulse s -1), provided the IR dose was sufficient and the radiation was applied directly to the cell. Examination of IR-evoked events during peak spontaneous [Ca 2+] i periods revealed a rapid drop in [Ca 2+] i, often restoring the baseline [Ca 2+] i concentration, followed by a transient increase in [Ca 2+] i. Cardiomyocytes were challenged with pharmacological agents to examine potential contributors to the IR-evoked [Ca 2+] i events. Three compounds proved to be the most potent, reversible inhibitors: (1) CGP-37157 (20 μm, n= 12), an inhibitor of the mitochondrial Na +/Ca 2+ exchanger (mNCX), (2) Ruthenium Red (40 μm, n= 13), an inhibitor of the mitochondrial Ca 2+ uniporter (mCU), and (3) 2-aminoethoxydiphenylborane (10 μm, n= 6), an IP 3 channel antagonist. Ryanodine blocked the spontaneous [Ca 2+] i transients but did not alter the IR-evoked events in the same cells. This pharmacological array implicates mitochondria as the major intracellular store of Ca 2+ involved in IR-evoked responses reported here. Results support the hypothesis that 1862 nm pulsed IR modulates mitochondrial Ca 2+ transport primarily through actions on mCU and mNCX.
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